In recent years, there are growing demands for biodegradable resin which can decompose in the natural environment, and molded products using the same from the viewpoint of environmental preservation. In particular, much efforts have been directed to investigations into biodegradable resins such as aliphatic polyester. Among others, polylactic acid generally has a high melting point (170 to 180° C.), and molded products of polylactic acid, which are transparent in general, have phased into practical use in some applications. General molded products of polylactic acid are, however, disadvantageous in that they have only a poor heat resistance, and will deform at temperatures above the glass transition point (Tg) which generally appears around 60° C. It is generally believed that the heat resistance up to 80° C. or around is necessary if they are going to be applied to enclosures or structural members of electric appliances. Extensive investigations are therefore in progress in order to make it possible to use them in applications in which heat resistance is required. It is to be understood that heat resistance referred herein means such as being sufficiently high in the rigidity (viscoelasticity) at around 80° C.
As one effort of raising the heat resistance of biodegradable polyester, addition of an inorganic filler is investigated. Heat-resistant mica is an example of the inorganic filler under investigation. This is aimed at improving mechanical characteristics of resin and hardening it through addition of a heat-resistant, hard inorganic filler, just like reinforcing concrete with steel bar. For example, talc, a kind of inorganic filler, is also known to promote crystallization of polylactic acid. It is therefore considered to be useful as a powerful nucleating agent in some applications. Problems may, however, arise in that a sufficient effect of the nucleating agent is obtainable only in an amount of addition of several tens of percent, and in that a larger amount of addition may embrittle the resin composition. The amount of addition of this level also results in whitening of the resin composition, so that transparency is not expectable.
Polylactic acid, a representative of biodegradable polyester, is a polymer capable of having a crystal structure, but general molded products thereof are likely to deform by heat due to its amorphous nature. One exemplary measure for raising the heat resistance under investigation is to crystallize polylactic acid by annealing during or after the molding process. This is aimed at hardening the resin through crystallization, and making it less causative of heat deformation. Addition of so-called nucleating agent is also under investigation for the purpose of promoting the crystallization in this case. Another known problem is that crystallization of polylactic acid by the general methods results in a crystal size of polylactic acid of only as large as microns to sub-millimeters, wherein the polylactic acid crystal per se is causative of scattering of light and makes the resin turbid and ruins the transparency.
The above-described nucleating agent can act as a primary crystal nucleus of a crystalline polymer, and can promote crystal growth of the crystalline polymer. In a broader sense, it is sometimes defined also as a substance for promoting crystallization of crystalline polymers. That is, also substances which increase the rate of crystallization per se are occasionally referred to as nucleating agents.
Addition of the nucleating agent of the former type to a resin results in micronization of the polymer crystal, and this successfully improves rigidity or transparency of the resin. Crystallization during molding can accelerate an overall rate (time) of the crystallization, and this also improves the moldability, which is typified by shortening of the molding cycle.
The above-descried effects can be observed in practical examples of other crystalline resins. For example, polypropylene (abbreviated as PP, hereinafter) has successfully been improved in the rigidity and transparency through addition of a nucleating agent, and the PP thus improved in the physical properties has been practiced in a variety of molded products. One known example of the nucleating agent is sorbitol-base substance, wherein a three-dimensional network created by the substance is considered as exhibiting a good effect, although the action mechanism thereof remains partially unclear. A nucleating agent of metal salt type for PP has already been put into practical use. Examples of this sort of nucleating agent include aluminum hydroxy-di(t-butyl benzoate), sodium bis (4-t-butylphenyl)phosphate, and sodium methylene-bis(2,4-di-t-butylphenyl)phosphate.
Investigations have also been made on nucleating agents for promoting crystallization of aliphatic polyesters. For example, Japanese Patent Application Publication No. HEI 10-158369 discloses a sorbitol-base substance. The substance has already been proven as a nucleating agent for PP, and is described that it can effectively act on polylactic acid when added thereto. Other methods of promoting crystallization of polyester through addition of the nucleating agent are disclosed typically in Japanese Patent Application Publications No. HEI 9-278991, No. HEI 11-5849, No. HEI 11-116783 and so forth. None of the investigations, however, have not successfully been practiced due to insufficient effects.
It is therefore an object of the present invention to provide a resin composition for molding, added with a nucleating agent suitable for promoting crystallization of polyester capable having a crystal structure, and in particular of biodegradable polyester. It is also an object of the present invention to provide a molded product containing the resin composition for molding thus improved in the crystallinity.